Question

Without doing any calculations, determine which solution in each pair is more acidic. a. \(0.0100 \mathrm{M}\) in \(\mathrm{HCl}\) and \(0.0100 \mathrm{M}\) in \(\mathrm{KOH}\) b. \(0.0100 \mathrm{M}\) in HF and \(0.0100 \mathrm{M}\) in \(\mathrm{KBr}\) c. \(0.0100 \mathrm{M}\) in \(\mathrm{NH}_{4} \mathrm{Cl}\) and \(0.0100 \mathrm{M}\) in \(\mathrm{CH}_{3} \mathrm{NH}_{3} \mathrm{Br}\) d. \(0.100 \mathrm{M}\) in \(\mathrm{NaCN}\) and \(0.100 \mathrm{M}\) in \(\mathrm{CaCl}_{2}\)

Short answer

a. HCl solution is more acidic. b. HF solution is more acidic. c. NH4Cl solution is more acidic. d. CaCl2 solution is less basic (more acidic) than NaCN solution.

Step-by-step solution

Understanding Acidity and Basicity

Recall that acids donate protons (H+) in solution, while bases accept protons. Strong acids and bases dissociate fully in solution, while weak ones do not. Hydrochloric acid (HCl) is a strong acid; potassium hydroxide (KOH) is a strong base. Hydrofluoric acid (HF) is a weak acid, while potassium bromide (KBr) is a salt that does not alter acidity. Ammonium chloride (NH4Cl) is a salt that forms a weak acid (NH4+) in solution, and methylammonium bromide (CH3NH3Br) is a salt that also forms a weak acid (CH3NH3+). Sodium cyanide (NaCN) is a salt that can form a weak base (CN-), while calcium chloride (CaCl2) is not directly acidic or basic.

Comparing Acidic Solutions

(a) HCl is a strong acid and will result in a more acidic solution than the strong base KOH. (b) HF is a weak acid and will result in a more acidic solution than KBr, which will not change the acidity. (c) Both NH4Cl and CH3NH3Br form weak acids upon dissociation, but CH3NH3+ is a weaker conjugate acid than NH4+, making NH4Cl more acidic. (d) NaCN will result in a slightly basic solution due to the formation of CN- anions, while CaCl2 will result in a neutral to slightly basic solution due to the presence of Ca2+, which can slightly increase pH if hydrolysis occurs; therefore, NaCN is likely to be less acidic (more basic) than CaCl2.

Key concepts

Acids and Bases

Understanding the nature of acids and bases is essential for predicting the acidity or basicity of solutions. Acids are substances that increase the concentration of hydrogen ions ((H^+)) when dissolved in water. Bases, on the other hand, are substances that increase the concentration of hydroxide ions ((OH^-)). The balance between these two types of ions in a solution determines its pH level—a measure of how acidic or basic a solution is.

For instance, when hydrochloric acid (HCl) dissolves in water, it releases (H^+) ions, making the solution acidic. Conversely, when potassium hydroxide (KOH) dissolves, it releases (OH^-) ions, which makes the solution basic. This simple yet fundamental concept helps in comparing the acidity in different solutions as seen in the exercise.

Strong and Weak Acids

The strength of an acid is determined by its ability to donate protons to the water it's dissolved in. Strong acids, like hydrochloric acid (HCl), dissociate completely in water, releasing all their hydrogen ions into the solution. This results in a high concentration of (H^+) ions and a correspondingly low pH, indicating a very acidic solution.

Weak acids, such as hydrofluoric acid (HF), don't fully dissociate in water. Instead, they release only some of their hydrogen ions, leading to a lower concentration of (H^+) ions and a higher pH than that of a solution with a strong acid. This variance in dissociation is the key to understanding the differences in acidity between solutions with strong and weak acids, and why in the given exercise, HF would result in a more acidic solution than a neutral salt like potassium bromide (KBr).

pH and Acidity

pH is a scale used to quantify the acidity of a solution, with lower values being more acidic and higher values more basic. It's a logarithmic scale based on the concentration of hydrogen ions ((H^+)): pH = -log([H+]), where [H+] is the molar concentration of hydrogen ions. The range goes from 0 (highly acidic) to 14 (highly basic), with 7 being neutral.

A solution with a pH less than 7 is considered acidic, and the lower the pH, the stronger the acid present. Substances like ammonium chloride (NH4Cl) can create an acidic environment by releasing (NH4+) ions which can donate protons, resulting in a more acidic solution when compared to other weak acids or salts that don’t affect the pH as significantly.

Conjugate Acids and Bases

Understanding conjugate acids and bases is pivotal when dealing with acid-base reactions. These are pairs of compounds that differ by the presence of one proton. When an acid donates its proton, it forms its conjugate base. Similarly, when a base accepts a proton, it forms its conjugate acid.

For example, after hydrochloric acid (HCl) donates a proton, it becomes the chloride ion (Cl^-), which is its conjugate base. Conversely, when ammonia (NH3) acts as a base and accepts a proton, it becomes ammonium (NH4+), its conjugate acid. The concept of conjugate acids and bases becomes especially useful when comparing the acidity of different salts in solution. As seen in the methylammonium bromide (CH3NH3Br) case from the exercise, its conjugate acid (CH3NH3+) is weaker than the conjugate acid from ammonium chloride (NH4Cl), which is NH4+, making the NH4Cl solution more acidic.